Terahertz Time-domain Spectroscopy Technique Research Articles

Fano resonance-integrated metal nanoparticles' enhanced sensing for pesticide detection.

The combined application of metasurface and terahertz (THz) time-domain spectroscopy techniques has received considerable attention in the fields of sensing and detection. However, to detect trace samples, the THz wave must still be enhanced locally using certain methods to improve the detection sensitivity. In this study, we proposed and experimentally demonstrated a fano resonance metasurface-based silver nanoparticles (FaMs-AgNPs) sensor. AgNPs can enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between the THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 µl, 20 µl, 30 µl and 40 µl, on the detection of acetamiprid. At 30 µl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml. The FaMs-AgNPs sensor has the advantages of a simple structure, easy processing, and excellent sensing performance, and has a great potential application value in the field of THz trace detection and other fields.

Identification of black plastics with terahertz time-domain spectroscopy and machine learning

Several optical spectroscopy and imaging techniques have already proven their ability to identify different plastic types found in household waste. However, most common optical techniques feasible for plastic sorting, struggle to measure black plastic objects due to the high absorption at visible and near-infrared wavelengths. In this study, 12 black samples of nine different materials have been characterized with Fourier-transform infrared spectroscopy (FTIR), hyperspectral imaging, and terahertz time-domain spectroscopy (THz-TDS). While FTIR validated the plastic types of the samples, the hyperspectral camera using visible and near-infrared wavelengths was challenged to measure the samples. The THz-TDS technique was successfully able to measure the samples without direct sample contact under ambient conditions. From the recorded terahertz waveforms the refractive index and absorption coefficient are extracted for all samples in the range from 0.4 to 1.0 THz. Subsequently, the obtained values were projected onto a two-dimensional map to discriminate the materials using the classifiers k-Nearest Neighbours, Bayes, and Support Vector Machines. A classification accuracy equal to unity was obtained, which proves the ability of THz-TDS to discriminate common black plastics.

In-situ indirect measurements of real-time moisture contents during microwave vacuum drying of beef and carrot slices using terahertz time-domain spectroscopy.

Moisture content (MC) is a critical quality indicator for food drying processing, however achieving in-situ non-destructive analyses of dynamic MC of products during processing is still a challenge. This study developed an in-situ indirect measurement method using Terahertz time-domain spectroscopy (THz-TDS) for real-time MC prediction of foods during microwave vacuum drying (MVD). During MVD, THz-TDS continuously sense the dynamic moisture vapour from the desiccator through a polyethene air hose. The obtained THz spectra were processed to calibrate MC loss prediction models using support vector regression, Gaussian process regression and ensemble regression. Then the MC was calculated using moisture loss prediction results. The best real-time MC prediction results for beef and carrot slices achieved R2 of 0.995, RMSE of 0.0162, and RDP of 22. The developed system provides a novel method for drying kinetics research during MVD and expands the applicability of the THz-TDS technique in the food industry.

Inner defect detection of glass fiber reinforced polymer sandwich panel using pulsed terahertz imaging based on smoothing and derivative

We conducted an experimental investigation to localize and detect internal defects in a sandwich panel made of glass-fiber-reinforced polymer (GFRP) using terahertz (THz) time-domain spectroscopy and signal processing techniques, which included smoothing and derivative analysis to enhance the defect detection performance. A simulated defect was created using a peel ply and scanned using a pulsed THz scanner with a reflection geometry specifically designed for this purpose. The reflected THz wave was analyzed to improve defect detection performance through time-domain signal processing using smoothing and derivative analysis. To quantify the performance of the proposed signal processing method, we introduced the Defect to Noise Ratio and evaluated its effectiveness accordingly. As a result, noise was removed and defect detection performance was enhanced, enabling the visualization of foreign insert defects in the GFRP sandwich panel.

Dopant profiling of ion-implanted GaAs by terahertz time-domain spectroscopy

We investigate terahertz time-domain spectroscopy (THz-TDS) as a non-destructive and non-contact technique for depth profiling of dopants in semiconductors. THz temporal waveforms transmitted through silicon-ion-implanted semi-insulating gallium arsenide substrates, as-implanted or post-annealed by rapid thermal annealing, were analyzed by assuming a multi-layered Gaussian refractive index profile in the ∼sub-micrometer-thick implantation region. The implantation energy and dosages in this work were 200 KeV, 1014, 5 × 1014, and 1015 ions/cm2, respectively. The average values of real (n) and imaginary (κ) parts of refractive indices of an as-implanted sample in the depth range of 0–800 nm are 5.8 and 0.7, respectively, at 0.5 THz and are 6.2 and 0.2, respectively, at 1 THz. On the other hand, the refractive index profile of the post-annealed samples displays a prominent Gaussian-like form, and peak refractive indices (n ∼ 25 and κ ∼ 32.7 at 0.5 THz and n, κ ∼17 at 1 THz) were found to be at the depth of 210 nm. Reconstructed dopant profiles in as-implanted, implanted, and post-annealed substrates were found to be in good agreement with measurements by secondary ion mass spectroscopy as well as simulation by the Monte Carlo method. We were also able to determine accurately the projected range (Rp), straggle (Rs), and concentration of dopants by the analysis of THz-TDS data. The spatial resolution, along the depth direction, of the THz-TDS technique for depth profiling of dopants was estimated to be as small as 8-nm. This work suggests the feasibility of using THz-TDS for nondestructive and non-contact diagnostics for profiling dopants in semiconductors.

An Improved Theoretical Model to Extract the Optical Conductivity of Two-Dimensional Material from Terahertz Transmission or Reflection Spectroscopy

The technique of terahertz time-domain spectroscopy (THz-TDS) enables us to simultaneously determine the real and imaginary parts of optical parameters. However, it is still a challenge to extract the optical parameters of a two-dimensional (2D) material (or an ultra-thin film) on a substrate accurately and flexibly for an arbitrary incident angle and different polarization. By treating a 2D material as a conductive boundary without thickness, we propose an improved theoretical model to extract the optical conductivity of the 2D material on a substrate from THz transmission or reflection spectroscopy. Importantly, the effects of wave polarization, incident angle, and multiple reflections in the substrate are considered in our model and the analytical formulae associated with the optical conductivity of the 2D material are provided. Furthermore, we verify the validation of our model based on the THz transmission and reflection experiments for mono- and few-layer MoS2 on sapphire substrates. These results not only are of practical significance for investigating the THz properties of 2D materials but can also be extended to the situations of ultra-thin films and/or incoherent detection such as Fourier transform infrared spectroscopy.

Measurement of Stress Optical Coefficient for Silicone Adhesive Based on Terahertz Time Domain Spectroscopy

The bonding structure is affected by environmental loads during use, causing internal stress in the adhesive layer, which leads to the debonding and expansion of the bonding layer. Therefore, it is important to accurately measure the stress distribution of the bonding layer to assess the life of the bonding structure. In this study, based on the transmission and reflection terahertz time-domain spectroscopy (THz-TDS) technique, the stress optical coefficients of a silicone adhesive were measured, and the calculation models of the transmission and reflection stress optical coefficients were derived. In the reflection calculation model, the caliper THz thickness measurement method was proposed to compensate for the thickness change of the silicone adhesive, under tensile stress. Under the transmission THz-TDS stress optical coefficient calculation model, the stress optical coefficient C of the silicone adhesive is 0.1142 ± 0.0057 MPa−1, and the stress optical coefficient C of the reflective system is 0.1135 ± 0.0051 MPa−1. The test results show that the reflective THz-TDS can also be used to measure the optical stress coefficient of the material, which compensates for the shortcomings of the traditional transmission measurement method, and lays a foundation for the characterization of the internal stress of the adhesive layer of the adhesive structure.

Temperature tunable electromagnetically induced transparency in terahertz metasurface fabricated on ferroelectric platform

The integration of active materials in terahertz (THz) metasurfaces is pivotal for the realization of functional device applications in diverse fields like sensing, imaging, communication, etc. In this context, ferroelectric materials endowed with tunable electro-optic properties have recently emerged as a novel candidate for achieving actively tuned THz metasurfaces. Here, we experimentally investigate temperature tuning of electromagnetically induced transparency (EIT) effects in a THz metasurface based on ferroelectric barium titanate (BaTiO3 (BTO)) thin film. We characterize tunable dielectric properties of the BTO thin film under variable temperatures (25 °C–100 °C) at THz frequencies by utilizing THz-time domain spectroscopy technique. Based on this aspect, we design a THz metasurface capable of displaying the EIT effect. THz transmissions through the metasurface sample are then probed for different applied temperatures. The EIT features undergo frequency shifts along with amplitude modulations owing to the temperature induced variations of the dielectric properties of the BTO thin film. A total red shift ∼27 GHz in EIT resonance dip is observed experimentally as the temperature increases from 25 °C to 100 °C. Therefore, we demonstrate utilities of ferroelectric platform toward the development of temperature tunable EIT metasurfaces.

THz Multi-Mode Q-Plate with a Fixed Rate of Change of the Optical Axis Using Form Birefringence.

We report the design, fabrication and experimental validation of a THz all-dielectric multi-mode q-plate having a fixed rate of change of the optical axis. The device consists of space-variant birefringent slabs manufactured by 3D printing using melt-extruded Acrylonitrile Butadiene Styrene (ABS). The desired form birefringence is analytically evaluated and experimentally measured by the THz time domain spectroscopy technique. The manufactured q-plate design is characterized using a polarization-sensitive imaging setup. The full electric field spatial maps are acquired from the beam propagating through the q-plate. The device enables the realization of both radial and azimuthal vector beams at discrete frequency intervals by controlling the space-dependent orientation of the ordinary and extraordinary axes in the transverse plane with a multi-mode sequence.

External bias dependent dynamic terahertz propagation through BiFeO3 film

Interactions of terahertz radiations with matter can lead to the realization of functional devices related to sensing, high-speed communications, non-destructive testing, spectroscopy, etc In spite of the versatile applications that THz can offer, progress in this field is still suffering due to the dearth of suitable responsive materials. In this context, we have experimentally investigated emerging multiferroic BiFeO3 film (∼200 nm) employing terahertz time-domain spectroscopy (THz-TDS) under vertically applied (THz propagation in the same direction) electric fields. Our experiments reveal dynamic modulation of THz amplitude (up to about 7% within 0.2–1 THz frequency range) because of the variation in electric field from 0 to 600 kV cm−1. Further, we have captured signatures of the hysteretic nature of polarization switching in BiFeO3 film through non-contact THz-TDS technique, similar trends are observed in switching spectroscopy piezoresponse force microscope measurements. We postulate the modulation of THz transmissions to the alignment/switching of ferroelectric polarization domains (under applied electric fields) leading to the reduced THz scattering losses (hence, reduced refractive index) experienced in the BiFeO3 film. This work indicates ample opportunities in integrating nanoscale multiferroic material systems with THz photonics in order to incorporate dynamic functionalities to realize futuristic THz devices.

Broadband terahertz characterization of graphene oxide films fabricated on flexible substrates

Graphene oxide (GO)fabricated on various flexible substrates (Kapton and lamination sheets) are studied with the help of terahertz time domain spectroscopy (THz-TDS) technique. In this work, GO films are grown using laser scribing technology, popularly names as Laser Induced Graphene (LIG) also, in the ambient conditions making the process cost-effective as well as user friendly. Terahertz transmission study, along with the sample images, suggests the presence of optical anisotropy in the fabricated GO films for different directions of laser scribing. It is further observed that THz transmissions are dominated by the Fabry Perot oscillations along with the wave guide resonance mode within the broadband THz characterization window. Several frequency dependent parameters (refractive index, conductivities, optical densities) essential for designing flexible terahertz devices are experimentally extracted for 0.3 THz to 2.0 THz frequency range. In near future, the outcome of this work can be helpful in integrating GO with flexible terahertz technology.

Study of weak intermolecular interactions and vibrational modes in hydroxybenzoic acid isomers using terahertz spectroscopy and density functional theory techniques

In this study, the terahertz (THz) absorption spectra of 2-, 3-, and 4-hydroxybenzoic acid molecules were recorded in the frequency range of 0.6–2.0 THz using the terahertz time-domain spectroscopy (THz-TDS) technique. The characteristic absorption peaks of these three hydroxybenzoic acid isomers were identified. The peak frequencies of the 2-, 3-, and 4-hydroxybenzoic acid molecules were significantly different from each other. Apart from experimental studies, theoretical studies using the density functional theory (DFT) technique were also conducted on the unit cells of the hydroxybenzoic acid molecules for simulating the experimental THz spectral features. The results were in good agreement with the experimentally obtained results. The vibrational characteristics and the weak intermolecular interactions of the three hydroxybenzoic acid isomers were analyzed by studying the potential energy distribution (PED) and using the energy decomposition analysis (EDA) technique based on the molecular force-field (FF; EDA-FF) methods to better understand the THz absorption spectra on a molecular level. The studies helped to systematically and comprehensively interpret the origin of the characteristic absorption peaks. The data obtained by studying the PED were used to explain the absorption peak mechanism in the theoretical spectra of the three isomers. The theoretical spectra were predicted using the DFT technique. The results obtained using the EDA-FF method revealed that the vibrational modes of the 2-hydroxybenzoic acid system are primarily the angle between the vector and the plane, while the vibrational modes of the 3-hydroxybenzoic acid and 4-hydroxybenzoic acid systems were primarily the dihedral angle (torsion). Results obtained using the EDA-FF analysis and the visual molecular dynamics (VMD) visualization methods revealed that the weak interactions exhibited by the 2-, 3-, and 4-hydroxybenzoic acid systems were primarily dispersive in nature. The results presented in this study revealed that the intermolecular hydrogen bonds (HBs) exhibited an obvious resonance response and played key roles in generating the low frequency vibrations in the THz region.

Measurement of Stress Optic Coefficient for Thermal Barrier Coating Based on Terahertz Time-Domain Spectrum

The residual stress introduced inside the thermal barrier coating (TBC) top coating during manufacturing and service processes is one of the main causes of thermal barrier failure. Therefore, a nondestructive and accurate measurement of the residual stress in top coating is essential for the evaluation of TBC life. The terahertz time-domain spectroscopy (THz-TDS) technique, which is based on the calibration or measurement of the stress optical coefficients of the measured materials, is applicable to the measuring of internal stress of nonmetal materials. In this work, to characterize the internal stress in TBC, the stress optic coefficient of the TBC top coating was measured by reflection-type THz-TDS. First, the mechanics model for the internal stress measurement in a TBC top coating was derived based on the photoelastic theory. Then, the THz time-domain spectra of TBC specimens under different loadings were measured in situ by a reflection-type THz-TDS system. Finally, the unimodal fitting, multimodal fitting and barycenter methods were used to carry out the data processing of the THz time-domain spectral-characteristic peaks. By comparing the processed results, the results using the barycenter method were regarded as the calibrated stress optical coefficient of the TBC due to the method’s sufficient accuracy and stability.

Recent Advances in Terahertz Time-Domain Spectroscopy and Imaging Techniques for Automation in Agriculture and Food Sector

Recent Advances in Terahertz Time-Domain Spectroscopy and Imaging Techniques for Automation in Agriculture and Food Sector

Characterization of Proton-Irradiated Polyaniline Nanoparticles Using Terahertz Thermal Spectroscopy

In this study, we investigated the changes in the molecular structure of polyaniline (PANI) nanoparticles illuminated by a proton beam using terahertz (THz) thermal spectroscopy based on the terahertz time-domain spectroscopy technique. PANI nanoparticles in water were exposed to a proton beam of 35 MeV energy with a particle fluence of 1013 particles/cm2. The photothermal properties of this solution of PANI nanoparticles were characterized using THz thermal spectroscopy. We measured the changes in the amplitudes of the reflected THz pulses to identify the variations in temperature induced by the photothermal effects of the PANI nanoparticle solution. The amplitude of a reflected THz pulse of the PANI solution not exposed to the proton beam increased when illuminated by an infrared light source, whereas that of THz signals of the PANI solution exposed to the proton beam hardly exhibited any changes. This implies that the molecular structure of PANI nanoparticles can be varied by a proton beam with a particle fluence above 1013 particles/cm2.

Rapid 3D-Imaging of Semiconductor Chips Using THz Time-of-Flight Technique

In this study, we developed a rapid three-dimensional (3D) time-of-flight imaging tool for inspection of packaged semiconductor chips, using terahertz (THz) time-domain spectroscopy techniques. A high-speed THz system based on the optical sampling by cavity tuning technique is incorporated with a 2-axis galvano scanner to deliver a scanning speed of more than 100 Hz/pixel with a signal-to-noise ratio larger than 20 dB. Through the use of the Hilbert transformation, we reconstruct the 3D structure of the packaged chip in a nondestructive manner. Additionally, the use of frequency-selective imaging allows us to manipulate image resolution; the higher resolution was obtained when monitored using the higher frequency component. Further, using phase information, we were able to detect and identify defects in the packaged chip, such as the delamination area and epoxy-rich regions.

Study on an artificial phenomenon observed in terahertz biological imaging.

Terahertz (THz) wave-based imaging of biological samples is an emerging but promising field. In the present work, we report an artificial phenomenon observed in imaging melanoma slices, which can lead to mistakenly interpretation of the experimental results. It was observed that a structure similar to but smaller than the sample contour appeared inside the melanoma slice image. The underlying mechanism of this phenomenon was then investigated both experimentally and theoretically. By imaging a regular standard sample (vinyl coverslip) with a THz time domain spectroscopy (THz-TDS) system and reconstructing its images at 0.8 and 1.2 THz, we can clearly observe the afore-mentioned artifacts. The experimental results are highly consistent with the simulations based on the Fresnel-Kirchhoff diffraction theory in which possible optical aberrations were incorporated. It can be concluded that this artifact was caused by the frequency-dependent diffraction of the sample edge. The work demonstrated here is essential for correct interpretation of the images obtained by the THz-TDS technique.

Terahertz single-scan extraction of the dynamic information in a layered deposition

Current terahertz time-domain spectroscopy (THz-TDS) theory and techniques are limited with respect to adequately and directly characterizing the layered structure information in dynamic deposition. We exploited a THz single-scan measurement and extraction method for the dynamic information in a layered deposition. The layer numbers, thickness and the deposition velocity can be simultaneously characterized with a single THz scan in a one-by-one deposition. Based on that, the THz-TDS can reveal the thickness information of a polycarbonate sheet with a continuously changing thickness. To demonstrate this, multiple layers of slices were measured. The layers were determined to have a special THz waveform that did not possess complete signals, while the THz-TDS clearly reflected the velocity and thickness of each sample in the process of superposition. The THz-TDS of the layered structures had multiple peaks. Thus, the layered structure information such as the insertion velocity and layers could be obtained for any condition. Consequently, we believe that a precise measurement carried with a THz single scan can help in improving online detection for deposition in the material science and petroleum industry.

Analysis of Flavonoid Compounds by Terahertz Spectroscopy Combined with Chemometrics.

Flavonoids are a large class of polyphenols widely distributed in plants in the free form or as glycosides, and they have antioxidation, antibacterial, antitumor growth, and other pharmacological effects. As an important active component of traditional Chinese medicine, they have high medicinal value and development prospects. In this work, the biomolecular properties of 10 common flavonoids, including baicalein, baicalin, apigenin, quercetin, naringenin, hesperetin, daidzein, genistein, puerarin, and gastrodin, are studied by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.2–2.5 THz. The results reveal that these flavonoids have different characteristic absorption peaks in the terahertz band. Moreover, the terahertz absorption characteristics of samples in the temperature range of 78–320 K are studied. The results show that the characteristic absorption peaks gradually increase with the decrease in temperature, and the frequency position of the absorption peak has a slight blue shift. Furthermore, qualitative identification and quantitative analysis of flavonoids are carried out by terahertz spectra combined with chemometrics. Specifically, a series of mixtures of three flavonoids with similar molecular structures under various concentrations are analyzed. The partial least-squares regression (PLSR) model and the artificial neural network (ANN) model are applied to quantitatively analyze the ternary mixture. The results confirm that the ANN model obtains the best predicted value, with the root-mean-square errors in the prediction set (RMSEP) of 1.27% for daidzein. In summary, the biomolecular properties of flavonoids are studied by the THz-TDS technique, and a rapid, effective, and nondestructive method for qualitative identification and quantitative analysis of flavonoids is provided. The results demonstrate that this method has potential application value in the detection of Chinese herbal medicine and has better referential significance for the study of other biomolecules, especially for isomers or similar molecular structures.

Substrate dependent terahertz response of monolayer WS2

We investigate experimentally the terahertz (THz) optoelectronic properties of monolayer (ML) tungsten disulfide (WS2) placed on different substrates using THz time-domain spectroscopy (TDS). We find that the THz optical response of n-type ML WS2 depends sensitively on the choice of the substrate. This dependence is found to be a consequence of substrate induced charge transfer, extra scattering centers, and electronic localization. Through fitting the experimental results with the Drude–Smith formula, we can determine the key sample parameters (e.g., the electronic relaxation time, electron density, and electronic localization factor) of ML WS2 on different substrates. The temperature dependence of these parameters is examined. Our results show that the THz TDS technique is an efficient non-contact method that can be utilized to characterize and investigate the optoelectronic properties of nano-devices based on ML WS2.